Solid-state image sensors, also known as imagers, were developed in the late 1960s and early 1970s primarily for television image acquisition, transmission, and display. An imager absorbs incident radiation of a particular wavelength (such as optical photons, x-rays, or the like) and generates an electrical signal corresponding to the absorbed radiation.
There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCD's), photodiode arrays, charge injection devices (CID's), hybrid focal plane arrays, and complementary metal oxide semiconductor (CMOS) imagers. Current applications of solid-state imagers include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detector systems, image stabilization systems, and other image acquisition and processing systems.
Solid-state imagers typically consist of an array of pixel cells. Each pixel cell contains a photosensor that produces a signal corresponding to the intensity of light impinging on the photosensor. When an image is focused on the array of pixel cells, the combined signals may be used, for example, to display a corresponding image on a monitor or otherwise used to provide information about the optical image.
The photosensors are typically phototransistors, photoconductors or photodiodes, in which the conductivity of the photosensor, or the charge stored in a diffusion region, corresponds to the intensity of light impinging on the photosensor. The magnitude of the signal produced by each pixel, therefore, is proportional to the amount of light impinging on the photosensor. Accordingly, it is important that all of the light directed to the photosensor impinges on the photosensor rather than becoming reflected or refracted. If light does not impinge on the correct photosensor, optical crosstalk between pixels may occur.
For example, optical crosstalk may exist between neighboring photosensors in a pixel array of a solid-state imager. In an idealized photosensor, a photodiode for example, light enters only through the surface of the photodiode that directly receives the light stimulus. In reality, however, light intended for neighboring photosensors also enters the photodiode, in the form of stray light, through the sides of the photosensor structure for example. Reflection and refraction within the photosensor structure can give rise to stray light, which is referred to as optical crosstalk.
Optical crosstalk can bring about undesirable results in images that are produced. The undesirable results can become more pronounced as the density of pixels in imager arrays increases, and as pixel size correspondingly decreases. The shrinking pixel sizes make it increasingly difficult to focus incoming light on the photosensor of each pixel.
Optical crosstalk can manifest as a blurring or reduction in contrast in images produced by a solid-state imager. In essence, crosstalk in an image sensor array degrades the spatial resolution, reduces overall sensitivity, causes color mixing, and leads to image noise after color correction. As noted above, image degradation can become more pronounced as pixel and device sizes are reduced. Furthermore, degradation caused by optical crosstalk is more conspicuous at longer wavelengths of light. Light at longer wavelengths penetrates more deeply into the silicon structure of a pixel cell, providing more opportunities for the light to be reflected or refracted away from its intended photosensor target.
One method to combat optical crosstalk is to employ a micro-lens array with the imager pixel array. The micro-lenses focus light onto respective photosensors, thereby increasing the amount of light energy impinging on each photosensor. Despite the use of micro-lens arrays, a large amount of incident light is still not directed efficiently onto the photosensors due to the geometry of the micro-lens array. As a result, the ability of a photosensor array to accurately reproduce an image varies between pixels across the array.
Another method to reduce optical crosstalk, in an imaging device, is the use of light shields. Light shields are formed in layers fabricated above the light-admitting surface through which the photosensor directly receives light stimulus. The light shield layers generally include metal and other opaque materials.
The light shields generally are formed as part of the uppermost layers of the solid-state imager array. Light shields have been formed, for example, in multi-level metal interconnect layers (e.g., Metal 1, Metal 2, or, if utilized, Metal 3 layers) of the photosensor's integrated circuitry. Light shields formed in such upper fabrication layers have inherent drawbacks, however. For example, metallization layers dedicated to light shielding are limited in their normal use as conductive connections for the imager array.
Additionally, light shields formed in upper device layers are separated from the light-admitting surface of the photosensor by several light transmitting layers. Moreover, the light shields are imperfect, and allow some light to pass into the light transmitting layers. Consequently, optical crosstalk still occurs through the light transmitting layers between the photosensor and the light shields. Having the light shields spaced apart from the surface of the photosensor can also increase light piping and light shadowing in the photosensors, leading to further errors in imager function.
Another method of reducing optical crosstalk uses optical waveguides, i.e., light tunnels. Optical waveguides are structures used for spatially confining light. For instance, optical waveguides can be used to reduce the detrimental affects associated with light shields such as light piping and light shadowing. Optical waveguides, however, are not currently used to focus light directly on the photosensor. Moreover, the optical waveguide structures that are currently employed, require additional processing steps, adding to the complexity and costs of imager fabrication.
Accordingly, solid-state imagers would benefit from a more efficient and effective optical waveguide structure that can focus light directly onto the photosensor, similar to a micro-lens. Of particular benefit would be a light tunnel that can be incorporated into current imagers to better mitigate optical crosstalk without added complexity to the manufacturing process.